1. Field of Invention
The disclosure herein relates to earth boring bits made from an alloy having high molybdenum content. More specifically this disclosure relates to earth boring bits comprised of an alloy having low carbon with high molybdenum. Yet, more specifically, the disclosure herein relates to earth boring bits having a low carbon content and a high molybdenum content, wherein the carbon content ranges up to 0.16% by weight and the molybdenum content exceeds 0.8% by weight.
2. Description of Prior Art
Drilling systems having earth boring drill bits are typically used in the oil and gas industry for creating wells drilled into hydrocarbon bearing substrata. Drilling systems typically comprise a drilling rig (not shown) used in conjunction with a rotating drill string wherein the drill bit is disposed on the terminal end of the drill string and used for boring through the subterranean formation. Drill bits typically are chosen from one of two types, either drag bits or roller cone bits. In FIG. 1, one example of a roller cone bit 10 is shown in perspective view. In this embodiment the roller cone bit 10 comprises a threaded connection 12 disposed on its upper most end for connection to the drill string. Formed on the lower most end of the threaded connection 12 is the bit body 14 which includes downwardly extending legs 18. Coaxially formed on each of the lower end of the legs 18 are roller cone cutters 20. A rolling cone bit 10 is designed such that the rolling cone cutters 20 rotate about their axis in conjunction with drill bit rotation. A series of cutting elements 22 are formed on the outer periphery of the rolling cone cutters 20. As is known, the cutting elements 22 contact the rock and subterranean formation and chip away individual pieces of the rock. An optional nozzle 16 may be included with the bit 10 for introducing a pressurized fluid, such as drilling fluid, during the cutting process. The drilling fluid (not shown) mixes with the cuttings and drill fluid pressure causes the cutting and fluid mixture to flow up the annulus formed between the drill string and the wellbore.
FIG. 2 illustrates a cross-sectional view of a portion of a drill bit 10a. In this embodiment, the cone cutter 20a is shown rotatingly mounted on a shaft 26. Bearings 28 are disposed on the outer circumference of a portion of the shaft 26 to aid in rotation of the cone cutter 20a about the shaft 26. As is known, weight on bit transferred to the rolling cone cutters 20a and cutting elements 22a via the threaded connection 12a produces localized stresses within sections of the body or leg section 18a. This is especially pronounced in sections such as the shoulder area 30, wherein the cross sectional area may be reduced in a portion of the bit 10a. These reduced areas therefore result in localized increases of stress which can lead to bit failure. Cyclic loading, either in the presence or absence of corrosive material, can lead to crack initiation and growth.
Roller cone earth boring bits are typically forged bodies comprised of a steel alloy, examples of known alloys include PS 30 and PS 55. Alloy PS 30 has a composition as follows: carbon 0.13%-0.18%, manganese 0.70%-0.90%, phosphorus 0.035% max, sulfur 0.040% max, silicon 0.15%-0.35%, nickel 0.70%-1.00%, chromium 0.45%-0.65%, molybdenum 0.45%-0.60%, and copper 0.35% max. For additional comparison purposes material properties of another alloy, referred to herein as PS 55, are provided in Table 1. The PS 55 composition includes carbon 0.15%-0.20%, manganese 0.70%-1.00%, phosphorus 0.025% max, sulfur 0.020% max, silicone 0.15%-0.35%, nickel 1.65%-2.00%, chromium 0.45%-0.65%, molybdenum 0.65%-0.80%, and copper 0.35% max.